This invention relates generally to sheet processing machines having a rotary sheet cylinder and a stationary feed table and more specifically it relates to a reciprocating auxiliary gripper system having a suction type gripper element.
Preferably, the sheet processing machine is a sheet fed rotary press in which the auxiliary gripper has the task to sieze a sheet located in an aligned position on the feed table, to accelerate the sheet and transfer the same to a clamping gripper on the periphery of the rotating feed cylinder.
In the GDR Pat. No. 122,239 a rocking preliminary gripper system is described which operates under the feed cylinder. The auxiliary gripper or sheet holding element in this known gripper system includes a movable gripper lip cooperating with a gripper stop movable on a circular path.
The disadvantage of this known gripper is the fact that its relatively large mass is subject to considerable inertial forces and moreover the control means for this gripper are prone to disturbing vibrations such as tangential or torsional vibrations. In order to keep the inertial forces low, the known clamping type gripper element is moved on a curved path having a relatively small radius. As a consequence, the sheet is moved on a strongly curved path which, however, is disadvantageous when extremely strong sheets (cartons) or extremely thin sheets (thin printing paper) are processed. The auxiliary gripper systems of this known construction are capable of performing their function in this case only when the plane of the feed table intersects the periphery of the subsequent feed cylinder.
From the German Pat. No. 1,163,342 a preliminary or auxiliary gripper is known, the sheet holding element of which is in the form of a suction element. It is true that this suction type gripper element removes the disadvantages of purely mechanical clamping grippers as far as the excessive inertial forces and disturbing vibrations of its control mechanism are concerned, but the sheet is still transferred from the feed table to the feed cylinder on a strongly curved path so that the aforementioned limits regarding the thickness of the processed paper sheets (cartons and thin printing papers) are still present. In addition, in the arrangement where the plane of the feed table intersects the feed cylinder, the alignment process of the subsequent sheet arriving on the feed table has the disadvantage of a relatively short time interval for such an alignment. This disadvantage results from the fact that the movable stops (front stops and top stops) upon aligning the sheet on the feed table must clear off the circular path of movement of the gripper element and can be swung up into their aligning position against the feed table only upon the passage of the front edge of the transported sheet past the point of intersection of the path of movement of the sheet and of the tip points of the marks. Consequently, in controlling the movement of the aligning stops, considerable rotary angles must be employed which are lost for the aligning process on the feed table.